There has been known a technique in which an ammonia generation part is provided which serves to heat and decompose urea by means of a heater to generate ammonia, so that urea water in the form of liquid and ammonia in the form of gas are supplied (for example, see a first patent document). However, because electric power is consumed so as to heat ammonia by means of the heater, energy is consumed too much. As a result of this, there is a fear that fuel economy or efficiency may get worse. In addition, it takes time to generate ammonia, so there is a possibility that much time may be taken until ammonia is supplied or the amount of ammonia to be supplied may be varied.